Debugger: Handle DW_TAG_GNU_template_template_param.

This is a GNU extension for identifying the case of a template parameter
that is itself a template. We don't currently make use of it, but this
allows us to parse/skip it correctly.

With this change, webcore is loadable/debuggable on x86-64 when built
with -gdwarf-4, with the caveat that ~3-4GB of RAM are currently required.